Radiation detector

ABSTRACT

A radiation detector comprises pixel electrodes which collect charges, a photoelectric converting layer which is provided on the pixel electrodes and which converts incident radiation into the charges, and which contains at least one or more kinds of heavy metal halide (AB n :A=heavy metal, B=halogen, n=either one of 1, 2, and 3) and at least one or more kinds of halogen (B 2 ) respectively, and an electrode layer which is provided on the photoelectric converting layer opposite to the pixel electrodes.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a Continuation Application of PCT Application No.PCT/JP2005/008712, filed May 12, 2005, which was published under PCTArticle 21(2) in Japanese.

This application is based upon and claims the benefit of priority fromprior Japanese Patent Application No. 2004-142800, filed May 12, 2004,the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a radiation detector provided with aphotoelectric converting layer in which incident radiation is convertedinto charges.

2Description of the Related Art

An active-matrix type plane detector largely attracts attention as animage detector for X-ray diagnosis of new generation. In the planedetector, an X-ray photographed image or a real time X-ray image isoutputted as digital signals by detecting irradiated X-rays Moreover,the plane detector has an extremely great expectation even in terms ofimage quality performance and stability since it is a solid detector.

As a first usage of practical use, the plane detector has been developedfor chest radiography or general radiography collecting a still imageunder a comparatively high dose, and commercialized in recent years. Inthe near future, commercialization is also expected for application infields of circulatory organ and digestive organ where it is necessary torealize a real time moving image of not less than 30frames per secondwith higher performance and under through-view dose. For the usage ofthe moving image, improvement of S/N ratio and a real time processingtechnology or the like of minute signals become important developmentitems.

Then, as for this kind of the plane detector, there are two kinds ofmethods of a direct method and an indirect method broadly classified.The direct method is a method in which photoconductive charges that aregenerated inside the body are directly converted into signal charges atthe inner part of an X-ray photoconductor layer of an a-Se or the likeby a high electric field, and these converted signal charges areaccumulated in a capacitor for charge accumulation. Moreover, in thedirect method, resolution characteristic which is nearly prescribed by apixel pitch of the active matrix can be obtained, since thephotoconductive charges generated by the incident X-ray are directly ledto the capacitor for charge accumulation by the high electric field.

On the other hand, in the indirect method, after the incident X-ray isreceived by a scintillator layer and once it is converted into thevisible light, the visible light is led to the capacitor for the chargeaccumulation by converting the light into the signal charges by an a-Siphotodiode or a CCD. Moreover, in the indirect method, the resolution isdeteriorated in accordance with optical diffusion and scattering untilthe visible light from the scintillator layer reaches the photodiode.

Further, as for the X-ray detector of the direct method, a TFT circuitboard as a photoelectric converting part at which the capacitor for theaccumulation, a thin film transistor (TFT), and a pixel electrode arerespectively installed is provided at every pixel arranged on asubstrate in a matrix shape. Then, for example, as shown in JapanesePatent Application Publication (KOKAI) No. 2003-209238(page 3-7, FIG.2), a constitution in which X-ray photoconductive films are laminated ona flattening layer including the pixel electrode of the TFT circuitboard as the photoconductive layer is known.

Namely, in this kind of the X-ray detector of the direct method, an“X-ray photoconductive material” is necessary in which the incidentX-ray is directly converted into the charge signals. Then, these X-rayphotoconductive materials are made as a kind of semiconductors. Further,as the main usage of an image detector which is an X-ray detector of thedirect method, the size just sufficient to be able to cover a human bodyis necessary since there are many cases of using information from theX-rays made being permeated through the human body for medical use. Forthese reasons, the X-ray detector having about 40 cm length on one sideis frequently used as an ordinarily used size.

At this time, if the X-ray detector of the direct method is attempted tobe realized, the X-ray photoconductive films must be formed uniformly onthe TFT circuit board having the size above 40 cm. Moreover, in order tofully detect the incident X-ray, the X-ray photoconductive films havingthe thickness of several hundreds μm are necessary even when usingmaterials which are constituted of heavy metals and which have a largespecific density. Namely, it is necessary to form a kind ofsemiconductor film having the size of even 40 cm of length on one side.

Further, as the X-ray detector of the direct method, amorphous selenium(a-Se) is used as the X-ray photoconductive material of the X-rayphotoconductive film. However, when using the a-Se as the X-rayphotoconductive material, a film-thickness of the X-ray photoconductivefilm of the a-Se is formed to be around 1 mm, and for example the a-Seis used as the X-ray photoconductive material by applying a strong biaselectric field of around 10 V/μm to both ends of the X-rayphotoconductive film in order to increase photoconductive charge formingrate per one piece of X-ray photon, in order to make the formedphotoconductive charges reach the pixel electrode without being trappedof the formed photoconductive charges by a defect level in the film, andin order to suppress the bias electric field and the diffusion of thecharges in the direction at right angles to the bias electric field.

Moreover, as the X-ray detector of other direct methods, a heavy metalcompound such as lead iodide (PbI₂) and mercury iodide (HgI₂) is used asthe X-ray photoconductive material. In this case, a heavy metal compoundsuch as these lead iodide (PbI₂) and mercury iodide (HgI₂) is formed onthe TFT circuit board in a film-like shape by vacuum vapor-deposition,or a solution is formed by mixing powders of these heavy metal compoundsinto a resin solution having charge transferability, and by drying thesolution after the solution is coated on the TFT circuit board, theX-ray photoconductive film is made.

However, when using the a-Se as the X-ray photoconductive material asdescribed above, there is a problem in stability since detectionsensitivity for converting the X-rays by the a-Se into the charges islow, and a recrystallization temperature is low. Further, in the a-Se,the film thickness of around 1 mmt is necessary since X-ray absorptionrate is low due to a small atomic number. Further, manufacturing costbecomes increasing since material efficiency is not so superior forforming the a-Se of the film thickness of around 1 mmt by avapor-deposition method which is a fabricating method of the a-Se.

Moreover, in composite coating formation with a highly sensitivephotoconductive material and an organic material binder, which isanother fabricating method when using the a-Se as ray photoconductivematerial, it is not easy to fully obtain original characteristic of thehighly sensitive photoconductive material since the a-Se is a compositewith the organic material binder. Further, when the highly sensitivephotoconductive material is formed by the vapor-deposition, thisformation has problems that unnecessary fabricating cost is chargedsince the material efficiency is not so superior, and it is not easy toobtain the X-ray photoconductive film of which characteristic issuperior.

BRIEF SUMMARY OF THE INVENTION

The present invention has been carried out by taking such points intoconsideration, and this object is to provide the radiation detectorwhich can improve the detection sensitivity and the stability of thephotoelectric converting layer.

According to an aspect of the present invention, there is provided aradiation detector comprising:

pixel electrodes which collect charges;

a photoelectric converting layer which is provided on the pixelelectrodes and which converts incident radiation into the charges, andwhich contains at least one or more kinds of heavy metal halide(AB_(n):A=heavy metal, B=halogen, n=either one of 1, 2, and 3) and atleast one or more kinds of halogen (B₂) respectively; and

an electrode layer which is provided on the photoelectric convertinglayer opposite to the pixel electrodes.

Additional advantages of the invention will be set forth in thedescription which follows, and in part will be obvious from thedescription, or may be learned by practice of the invention. Theadvantages of the invention may be realized and obtained by means of theinstrumentalities and combinations particularly pointed out hereinafter.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate embodiments of the invention, andtogether with the general description given above and the detaileddescription of the embodiments given below, serve to explain theprinciples of the invention.

FIG. 1 represents a graph showing impurity-containing pulse sensitivitydependency of a photoelectric converting layer of a radiation detectorof one embodiment of the present invention;

FIG. 2 represents a graph showing impurity-containing dark-currentdependency of the photoelectric converting layer of the radiationdetector;

FIG. 3 represents an exemplary cross-sectional view to show theradiation detector;

FIG. 4 represents an exemplary constitutional drawing to show theradiation detector;

FIG. 5 represents an exemplary top plan view to show the radiationdetector;

FIG. 6A represents a drawing to show characteristics of a photoelectricconverting layer of the radiation detector;

FIG. 6B represents a drawing succeeding to FIG. 6A to showcharacteristics of a photoelectric converting layer of the radiationdetector;

FIG. 6C represents a drawing succeeding to FIG. 6B to showcharacteristics of a photoelectric converting layer of the radiationdetector;

FIG. 6D represents a drawing succeeding to FIG. 6C to showcharacteristics of a photoelectric converting layer of the radiationdetector;

FIG. 7 represents a drawing to show peaks of intensity of respectivemeasurement documents of the photoelectric converting layers of theradiation detector;

FIG. 8 represents a graph to show peaks of intensity of respectivemeasurement document of the photoelectric converting layer of theradiation detector;

FIG. 9A represents a drawing to show peaks of intensity of respectivemeasurement documents of the photoelectric converting layer of theradiation detector;

FIG. 9B represents a drawing succeeding to FIG. 9A to show peaks ofintensity of respective measurement documents of the photoelectricconverting layer of the radiation detector; and

FIG. 10 represents a graph to show peaks of intensity of respectivemeasurement document of the photoelectric converting layer of theradiation detector.

DETAILED DESCRIPTION OF THE INVENTION

The constitution of one embodiment for carrying out the preferredembodiment of a radiation detector of the present invention is explainedwith reference to drawings of FIG. 1 to FIG. 5 as follows.

In FIG. 3 to FIG. 5, the X-ray detector 1 of the direct method is anX-ray plane sensor of straight line conversion type as an X-ray planardetector detecting an image of X-ray which is a radiation. The X-raydetector 1 is a detector for the X-ray, and a halogen compound ofespecially high sensitivity is used for X-ray light electroconductivematerial. Moreover, the X-ray detector 1 is provided with aphotoelectric converting substrate 2 as a TFT circuit board as shown inFIG. 3 and FIG. 4. The photoelectric converting substrate 2 is an activematrix photoelectric converting substrate as a TFT circuit board.

Then, the photoelectric converting substrate 2 has a glass substrate 3which is an insulating substrate such as glass having translucency. On asurface which is one main surface of the glass substrate 3, a pluralityof nearly rectangular photoelectric converting part 4 which functions asa photosensor are arranged in matrix. Then, on the surface of the glasssubstrate 3, a plurality of pixels 5 of respectively the same structureare installed by the respective photoelectric converting parts 4. Theserespective pixels 5 are thin film element pixels arranged and formedtwo-dimensionally at a respectively prescribed pitch P in the rowdirection which is the lateral direction in FIG. 5 and in the columndirection which is the longitudinal direction in FIG. 5.

Then, at these respective pixels 5, pixel electrodes 6 of nearlyL-shaped flat plates as current collecting electrodes to collectelectric signals and signal charges are provided at these respectivepixels 5. These respective pixel electrodes 6 are provided respectivelyat a pixel unit, namely at the center part of the respective pixels 5 onthe surface of the glass substrate 3. Here, these pixel electrodes 6 arefilm-formed for example by an Indium-Tin Oxide(ITO) transparentconducive film or an aluminum (Al) thin film by a sputtering method andan electron beam (EB) vapor-deposition method or the like. Further,after film-forming the ITO film or the aluminum film which become thesepixel electrodes 6, these pixel electrodes 6 are separated andconstituted at every pixel unit by a Photoelectrophoretic imaging(PEP)process including an etching process.

Further, thin film transistors (TFT) 7 as switching element partsconstituting switching parts are electrically connected to theserespective pixel electrodes 6. As for these respective thin filmtransistors 7, at least one portion is constituted of amorphous silicon(a-Si) as an amorphous semiconductor which is a semiconductor materialhaving a crystallinity. Further, these respective transistors 7 storeand release the charges based on a potential difference detected at thepixel electrodes 6. Moreover, these respective thin film transistors 7are respectively provided at the respective pixels 5.

Moreover, at these respective pixels 5, accumulating capacitors 8 on arectangular flat plate are provided which are accumulating elements as acharge accumulating capacity part to accumulate the signal chargesdetected at the pixel electrodes 6. These accumulating capacitors 8 areoppositely arranged under the pixel electrodes 6. Here, the thin filmtransistors 7 have a gate electrode 11, a source electrode 12, and adrain electrode 13, respectively. The drain electrode 13 is electricallyconnected to the pixel electrodes 6 and the accumulating capacitors 8,respectively.

Further, on one side edge along the row direction on the surface of theglass substrate 3, a high-speed signal processing part 14 which is acontrol circuit as a driver circuit of a slender elongated rectangularflat plate state to control an operation state of the respective thinfilm transistors 7, for example ON and OFF of the respective thin filmtransistors 7 is mounted. The high speed processing part 14 is a linedriver as a signal processing circuit to control reading out of thesignal and to treat the read-out signal. Then, the high-speed signalprocessing part 14 has the longitudinal direction along the columndirection on the surface of the glass substrate 3, and is arranged in afolding-back state to the rear surface side of the glass substrate 3.Namely, the high-speed signal processing part 14 is mounted oppositelyto the rear surface side of the glass substrate 3.

Then, one end of each control line 15 is electrically connected to thehigh-speed processing part 14. These respective control lines 15 arewired along the row direction of the glass substrate 3 and arrangedbetween the respective pixels 5 on the glass substrate 3. Further, theserespective control lines 15 are electrically connected to the gateelectrodes 11 of the thin film transistors 7 constituting the respectivepixels 5 of the same row respectively.

Moreover, on the surface of the glass substrate 3, a plurality of datalines 16 along the column direction of the glass substrate 3 are wired.These respective data lines 16 are arranged between the respectivepixels 5 on the glass substrate 3. Then, these respective data lines 16are electrically connected to the respective source electrodes 12 of thethin film transistors 7 constituting the pixels 5 of the same column.Namely, these respective data lines 16 makes image data signals receivedfrom the thin film transistors 7 constituting the pixels of the samecolumn.

Then, one end of each data line 16 is electrically connected to thehigh-speed signal processing part 14. Further, a digital imagetransmission part 17 is electrically connected to the high-speed signalprocessing part 14. The digital image transmission part 17 is mounted ina state of being led outside the photoelectric converting substrate.

On the other hand, as shown in FIG. 3, the thin film transistor 7 andthe accumulating capacitor 8 are formed at each pixel 5 on the surfaceof the glass substrate 3. Here, each of the thin film transistor 7 isprovided with the gate electrodes 11 of an island state formed on theglass substrate 3. Then, an insulating film 21 is laminated and formedon the glass substrate 3 including the gate electrodes 11. Thisinsulating film 21 covers the respective gate electrodes 11.

Moreover, a plurality of semi-insulating films 22 of island state arelaminated and formed on this insulating film 21. The semi-insulatingfilms 22 are arranged opposite to the respective gate electrodes 11, andcover these respective gate electrodes 11. Namely, each semi-insulatingfilm 22 is provided on the gate electrode 11 via the insulating film 21.Further, on the insulating film 21 including the semi-insulating films22, the source electrodes 12 and the drain electrodes 13 are formedrespectively. In each thin film transistor, the source electrode 12 anddrain electrode 13 are mutually insulated and not electricallyconnected. Moreover, the source electrode 12 and drain electrode 13 areprovided at both sides on the gate electrode 11, and the respective oneend parts of these source electrode 12 and drain electrode 13 arelaminated on the semi-insulating films 22.

Then, as shown in FIG. 5, the gate electrode 11 of each thin filmtransistor 7 is electrically connected to the common control line 15together with the gate electrodes 11 of the other thin film transistors7 positioned in the same row. Further, the source electrode 12 of eachthin film transistor 7 is electrically connected to the common data line16 together with the source electrodes 12 of other thin film transistors7 positioned in the same column.

On the other hand, each accumulating capacitors 8 is provided with thelower part electrode 23 of the island state formed on the glasssubstrate 3. The insulating film 21 is laminated and formed on the glasssubstrate 3 including the lower part electrodes 23. This insulating film21 is extended from on the gate electrode 11 to on the lower partelectrode 23 of each thin film transistor 7. Further, the upper partelectrodes 24 of the island state are laminated and formed on theinsulating film 21. Each upper part electrode 24 is arranged opposite tothe lower part electrode 23, and covers the lower part electrode 23.Namely, each upper part electrodes 24 is installed on the lower partelectrodes 23 via the insulating film 21. Then, each of the drainelectrode 13 is laminated and formed on the insulating film 21 includingthe upper part electrode 24. As for the drain electrode 13, the otherend part is laminated on the upper part electrode 24, and electricallyconnected to the upper part electrode 24.

Further, on the insulating film 21 including the semi-insulating film22, the source electrode 12, and the drain electrode 13 of therespective thin film transistors 7 and including the upper partelectrode 24 of the respective accumulating capacitors 8 respectively, aflattening layer 25 as the insulating layer is laminated and formed. Theflattening layer 25 is constituted of a resin, and in one part of theflattening layer 25, a through hole 26 which is a contact hole as acommunicating part communicated with the drain electrode 13 of the thinfilm transistor 7 is opened and formed. Then, the pixel electrodes 6 arelaminated and formed on the flattening layer 25 including the throughholes 26. Accordingly, each pixel electrode 6 is electrically connectedto the drain electrode 13 of the thin film transistor 7 via the throughhole 26. Moreover, the thin film transistor 7 is provided at the lowerlayer of the pixel electrode 6.

Further, on the flattening layer 25 including the pixel electrodes 6, aphotoconductive layer 31 as a radiation photoelectric converting layerto change an incident X-ray L into charges is film-formed and laminated.The photoconductive layer 31 is an X-ray photoconductive film as anX-ray photoelectric converting film. Here, the pixel electrodes 6 areprovided under the photoconductive layer 31 which is the side opposed tothe X-ray L incident into the photoconductive layer 31 in a state ofdirect contact with this photoconductive layer 31. In other words, thepixel electrodes 6 are provided on the opposite side of the directionside where the X-ray L is incident against the photoconductive layer 31.Namely, the pixel electrodes 6 are provided at the lower face of thephotoconductive layer 31 positioned on the opposite side to the sidewhere the X-ray L is incident against the photoconductive layer 31.

Then, this photoconductive layer 31 has a film thickness of 0.3 mmt, andis constituted of an X-ray photoconductive material which is thephotoconductive material to convert the incident X-ray L into theelectric signal.

Here, the photoconductive layer 31 is one in which limitations are addedregarding components, composition ratio, specified impurities, andeffects of quantitative contents of these specified impurities, etc. ofthe X-ray photoconductive material constituting the photoconductivelayer 31. Concretely, as the X-ray photoconductive material, at leastone kind or more of heavy metal halide (AB_(n):A=heavy metal, B=halogen,n=either one of 1, 2, and 3) and at least one kind or more of halogen(B₂) are contained, respectively.

Namely, the photoconductive layer 31 is composed of at least one kind ormore of heavy metal halide (AB_(n):n=1, 2, or 3) and at least one kindor more of halogen (B₂) as the X-ray photoconductive material.Concretely, in this X-ray photoconductive material, as the heavy metalhalide, at least one kind or more of lead iodide (PbI₂), mercury iodide(HgI₂), indium iodide (InI), thallium iodide (TlI), and bismuth iodide(BiI₂) are contained. Moreover, in the X-ray photoconductive material,iodine (I) is contained as halogen.

Further, the photoconductive layer 31 contains at least one kind or moreof heavy metal (A) as the X-ray photoconductive material. Namely, thephotoconductive layer 31 contains, for example lead (Pb) element andiodine (I) element as the main components. At this time, in thephotoconductive layer 31, besides the lead element, mercury (Hg) elementor the like is configured to be used and contained. Further, in thephotoconductive layer 31, besides the iodine element, other halogenelement is configured to be used and contained also.

Further, the photoconductive layer 31 is constituted for the purposethat a molar composition ratio (B/(nA)) of halogen (B) and heavy metal(A) constituting the heavy metal halide in the photoconductive layer 31become 0.9or more and 1.1or less. Further, the photoconductive layer 31is constituted for the purpose that respective contents contained in thephotoconductive layer 31 of elements belonging to neighboring periodicfamilies to the heavy metal element and of elements neighboring to theelements belonging to the same periodic families in the neighboringperiods before and after the heavy metal element in the periodic tablebecome 10wtppm or less as impurities.

Here, the photoconductive layer 31 is adhered and formed on theflattening layer 25 including the pixel electrodes 6 for example by ametal such as indium (In) and a conductive resin or the like. Moreover,the photoconductive layer 31 is configured to be adhered on theflattening layer 25 including the pixel electrodes 6 by thermal pressurebonding.

Further, on the photoconductive layer 31, a bias electrode layer 32which is a thin film electrode as an electrode layer is laminated andformed. The bias electrode layer 32 is a bias electrode film laminateduniformly over the whole photoelectric converting parts 4. Moreover, thebias electrode layer 32 is provided on the photoconductive layer 31opposite to the pixel electrodes 6. Then, as for the bias electrodelayer 32, for example an ITO film or an aluminum film is film-formed bya sputtering method.

In other words, as for the bias electrode layer 32, for example an ITOtransparent conductive film or an aluminum thin film is film-formed bythe sputtering method or an electron beam vapor-deposition method or thelike. Namely, this bias electrode layer 32 is integrally formed so thata bias electric field can be formed between each pixel electrode 6 byapplying a common bias voltage to the pixel electrode 6 of each pixels5.

Next, an action of the above described one embodiment is explained.

Firstly, the X-ray L is incident into the photoconductive layer 31, andthe incident X-ray L by this photoconductive layer 31 is converted intosignal charges which are electric signals. At this time, the signalcharges are carried and moved to the pixel electrode 6 by the biaselectric field formed between the bias electrode layer 32 and the pixelelectrode 6, and accumulated in the accumulating capacitors 8 via thedrain electrode 13 and the like from this pixel electrode 6.

On the other hand, reading-out of signal electric charges accumulated inthe accumulating capacitor 8 is controlled by a high-speed signalprocessing part 14 sequentially, for example, by every row of the pixelunit 12 (in the lateral direction of FIG. 5).

At this time, an ON signal of, for example 10V is added to each of thegate electrodes 11 of the pixel unit positioned at a first row from thehigh-speed signal processing part 14 through a first data line 16,thereby each of the thin film transistors 7 of the pixel unit at thefirst row is made to have an ON state.

At this time, the signal charges accumulated in each of the accumulatingcapacitors 8 of the pixel unit of the first row are outputted as theelectric signals from the drain electrode 13 to the source electrode 12.Then, the electric signals outputted to each source electrode 12 areamplified by the high-speed signal processing part 14.

Further, these amplified electric signals are added to a digital imagetransmission part, and after they are converted into series signals,they are converted into digital signals, and sent to a signal processingpart of the next stage which is not illustrated.

Then, when reading-out of the electric charges of the accumulatingcapacitors 8 of the pixel unit positioned at the first row is finished,an OFF signal of, for example −5V is added to the gate electrodes 11 ofthe pixel unit of the first row from the high-speed signal processingpart 14 through a first data line 16, thereby the respective thin filmtransistors 7 of the pixel unit of the first row is made to have an OFFstate.

Afterwards, above mentioned actions are sequentially carried out to thepixel unit in the second row in the order. Then, the signal chargesaccumulated in the accumulating capacitors 8 of all the pixel unit areread out, converted into digital signals and are outputted in the order,thereby the electric signal corresponding to an X-ray image plane isoutputted from the digital image transfer part 17.

As mentioned above, according to one embodiment, since halogen inaddition to heavy metal halide is made to be contained in thephotoconductive layer 31, variations of the dark-current characteristic,the sensitivity characteristic, and the afterimage characteristic of thephotoconductive layer 31 by irradiation of X-ray L are suppressed, and astable photoconductive layer is made. At this time, when a surplushalogen is contained in this photoconductive layer 31, it is consideredthat an effect is occurring that suppresses dissociation of halogen inthe heavy metal halide crystal structure which is apt to occur duringthe irradiation of the X-ray L, and that suppresses generation ofcrystal defects accompanied with this dissociation of halogen.

Namely, it is considered that since the dissociation of halogen from thecrystal structure of the photoconductive layer 31 yields the defectlevel in the photoconductive layer 31 that causes a deep trap of theelectric charge influencing on the dark-current characteristic,sensitivity characteristic and afterimage characteristic, suppression ofthe occurrence of this defect is extremely effective for a stable actionof the X-ray detector 1. However, if this surplus halogen exists in thisphotoconductive layer 31 too much, halogen is precipitated at grainboundary of this photoconductive layer 31, and the afterimage islengthened since conductivity between the minute crystals of thephotoconductive layer 31 is obstructed, the sensitivity characteristicof this photoconductive layer 31 as a whole is obstructed, andespecially the afterimage characteristic is obstructed greatly.

From such reasons, it is desirable that the surplus halogen in thephotoconductive layer 31 is within a proper range against thestoichiometry of the heavy metal halide. Concretely, as the total amountobtained by adding the heavy metal halide (AB_(n)) and halogen (B₂),namely as the ratio in the total between halogen element (B) and heavymetal element (A), roughly the range of around B/(nA)≦1.1is desirable.

Further, the heavy metal element is further contained in thephotoconductive layer 31 by making the heavy metal halide, halogen, andthe heavy metal respectively contained in this photoconductive layer 31.For that reason, effective charge mobility of the photoconductive layer31 increases, and the improvement in collection efficiency of X-rayphotoconductive charges in the photoconductive layer 31 leads to theimprovement in sensitivity. At this time, as for characteristic having atrade-off relationship with that when surplus heavy metal elements areincreased in the photoconductive layer 31, dark-current is increased.Accordingly, it is desirable that surplus heavy metals are within amoderate range against the stoichiometry of the heavy metal halide.Namely, the heavy metal halide (AB_(n)), halogen (B₂) and heavy metalare summed up, and roughly the range of around 0.9≦B/(nA) is desirableas the ratio between the heavy metal element (A) and the halogen element(B).

Moreover, as for impurities contained in the photoconductive layer 31,it is undesirable that the element belonging to families neighboring tothe heavy metal element constituting the heavy metal halide, andfamilies neighboring to the periods before and after the heavy metalexist. This comes to compose donor level and acceptor level in thephotoconductive layer by means that these impurities replace the heavymetal element of the heavy metal halide. Accordingly, this is becausethat the dark-current increases by lowering resistivity after the vacantlevel density of a conduction band and the valence electron density of avalence band are reduced, while there is a strong possibility ofdecreasing change of the film resistivity, namely sensitivitycharacteristic at the time of X-ray L irradiation.

Concretely, when lead iodide is used for the heavy metal halide, as aconcrete example of the undesirable impurities, thallium of family 3Bneighboring to lead (Pb) of family 4B, bismuth of family 5B, and indiumwhich is of family 3B and antimony (Sb) or the like of family 5B beforeone period in the periodic table are corresponding. Accordingly, metalimpurity elements are often contained at the ratio of dozens of ppm inraw materials of the heavy metal halide and the film-formedphotoconductive layer 31 using the materials, but these undesirableimpurity elements are necessary to be suppressed to at most 10wtppm orless.

Namely, as a conventional photoconductive layer, a film in whichamorphous selenium (a-Se) has been vapor-deposited into thephotoconductive layer, a film on which a mixture has been coated after abinder such as epoxy compound is mixed into a highly sensitivephotoconductive material such as heavy metal halide, and a film whichhas been formed by vapor-deposition of a highly sensitivephotoconductive material single body or by single crystal growth or thelike have been used. On the contrary, as one embodiment as describedabove, the detection sensitivity becomes larger than that of thephotoconductive layer of an a-Se film by single-digit or more by makingheavy metal halide, halogen and heavy metal respectively contained inthe photoconductive layer 31. Further, the amount of X-ray absorptionlike the photoconductive layer of the a-Se film can be secured becauseof having film-thickness which is a half or less of the photoconductivelayer of the a-Se film.

Accordingly, the detection sensitivity and photoconductivecharacteristics of a direct type X-ray detector 1 can be stabilized.Thus, since the X-ray detector 1 has the high sensitivity, the smalldark-current, and excellent afterimage characteristic, and whereinchanges in these characteristics especially by the irradiation of theX-ray L can be suppressed to be extremely small, this can be made tohave excellent stability and reliability.

Moreover, explanation on the X-ray detector 1 for detecting the X-ray Lhas been made in the above-mentioned respective embodiments, but aradiation detector can be used by being made to correspond to the X-raydetector, for example even if the detector is the radiation detector fordetecting various kinds of radiations besides the X-ray L for examplesuch as γ-rays. Moreover, like an area sensor, etc., pixels 5 in whichthin film transistors 7 and pixel electrodes 6 are formed on a glasssubstrate 3 of a photoelectric converting part 4 in a two-dimensionallymatrix respectively along the longitudinal direction and the lateraldirection, but the pixels 5 may be provided one-dimensionally on theglass substrate 3 of the photoelectric converting part 4 in the case ofa line sensor or the like.

Moreover, even if the photoconductive layer 31 is converted to be ascintillator layer in which the incident X-ray L is converted intovisible light, and the pixel electrode 6 is converted to be a photodiodeinto which the visible light converted by the scintillator layer isconverted into signal charges, the same action effect as that of theX-ray detector 1 in the above-mentioned respective embodiments can beplayed. Further, this can be used as the X-ray detector 1 by being madeto cope with the detector using the thin film transistor 7 constitutedof an amorphous semiconductor, a crystalline semiconductor, and apolycrystalline semiconductor.

EXAMPLE

Next, one example of the present invention will be explained.

Firstly, as for the photoconductive layer 31 of a conventional heavymetal halide obtained by film-forming lead iodide (PbI₂) as rawmaterials on a flattening layer 25 including the pixel electrodes 6 by ageneral vacuum vapor-deposition method or the like, the halogen elementhaving a high vapor pressure is apt to be rather short inevitably whenthe heavy metal and halogen are decomposed and vapor-deposited onto thephotoelectric converting substrate 2. Especially, a tendency in whichthe halogen element runs short becomes stronger as a temperature of thephotoelectric converting substrate 2 is risen in order to improvecrystallinity in the film of the photoconductive layer 31.

Thus, in order to carry out trial manufacturing acharacteristic-improved film containing halogen, or halogen and theheavy metal in the photoconductive layer 31, the photoconductive layer31 which is vapor-deposition film has been formed by taking-invapor-deposition atmosphere in iodine (I) vapor, and by appropriatelychanging temperature and vapor-deposition speed of respectivephotoelectric converting substrates 2. At this time, the film thicknessof the photoconductive layer 31 has been united to around 100 μm. Then,by forming an ITO film on the photoconductive layer 31 by a sputteringmethod, and by making them as a bias electrode layer 32, the sensitivityand dark-current characteristics of the bias electrode layer 32 havebeen compared.

Moreover, as analysis of the photoconductive layer 31, a crystalstructure constituting the photoconductive layer 31 has been analyzed byusing an X-ray diffraction analyzer (XRD), and a composition ratio ofmain constituting elements has been analyzed by using an energydispersion type X-ray micro-analyzer (EDX). At this time, materials,analyzed results, and characteristics (sensitivity, dark-current andafterimage) at 25°C. of respective photoconductive layers 31 arerespectively shown in FIG. 6A to FIG. 10.

As a result, in the case lead iodide (PbI₂) is used as the X-rayphotoconductive material in the photoconductive layer 31, and in thecase not only lead iodide but also iodine (I) are detected in thephotoconductive layer 31 from the results analyzed by the X-raydiffraction analyzer as shown in FIG. 6A to FIG. 6D, changes of thedark-current characteristic and the sensitivity characteristic beforeand after the irradiation of the X-ray L are suppressed smaller. At thistime, irradiation conditions of this X-ray L are the results ofirradiating the X-ray L for 10seconds under conditions of 3mR/frame,pulse width 16msec, and 30frames/sec, and measuring the sensitivity anddark-current characteristics after the irradiation of the X-ray L afterone second from finishing of the irradiation.

Further, it is found that the afterimage characteristic by the X-raydetector 1 is also excellent. Namely, it is considered that excellenceof the afterimage characteristic (afterimage is less) is connected to amerit having small dark-current change (increase) after the irradiationof the X-ray L. However, since adverse effect to the afterimagecharacteristic and the sensitivity characteristic are produced whensurplus iodine contained in the photoconductive layer 31 is too much, asthe composition ratio of I/Pb, the range up to 2.2, namely aroundI/(2Pb)≦1.1is preferable. At this time, small characteristic change inthe photoconductive layer 31 against the irradiation of the X-ray L isan extremely important characteristic for the stable operation of theX-ray detector 1.

Moreover, lead iodide and iodine are detected in the photoconductivelayer 31 in the analysis by the X-ray diffraction analyzer, and as for asample in which the composition ratio of I/Pb is 2or less in theanalysis by the energy dispersion type X-ray micro-analyzer, free leadis considered to be present in the photoconductive layers 31. At thistime, the sensitivity characteristic of this photoconductive layer 31 isimproved. However, even in this case, the dark-current comes to beextremely increased since surplus lead contained in this photoconductivelayer 31 is too much. For that reason, as the composition ratio ofiodine (I)/lead (Pb) in the photoconductive layer 31, the range up to1.8, namely around 0.9≦I/(2Pb)≦1. 1is preferable.

At this time, as for the effect of impurities in the photoconductivelayer 31, even in the case elements such as iron (Fe), aluminum (Al),and silver (Ag) are contained as shown in FIG. 1, there is no adverseeffect especially to the sensitivity characteristic, and in the case,for example bismuth (Bi) is contained in the photoconductive layer 31,the sensitivity characteristic becomes significantly deteriorated.Moreover, also regarding the adverse effect to the dark-currentcharacteristic, the dark-current characteristic is larger than thesensitive characteristic by about single-digit in the photoconductivelayer 31 in which bismuth is contained as shown in FIG. 2.

Namely, as shown in FIG. 6A to FIG. 10, when elements belonging to thefamilies neighboring to lead, belonging to the family before and afterthis lead, and belonging to families neighboring to the family in theperiodic table, such as thallium (Tl), indium (In), and tin (Sn) werecontained by not less than 10 wtppm, characteristic deteriorations suchas reduction in the sensitivity and increase in the dark-current havebeen significantly found. Accordingly, without being limited to leadiodide, the same effect regarding the impurities has been observed evenin mercury iodide (HgI₂), indium iodide (InI), bismuth iodide (BiI₃),and thallium iodide (TlI).

As these results, the improved results of the photoconductive layer 31as described above have been similar even if dose and irradiationcondition of other X-rays were changed. Moreover, even if the X-rayphotoconductive materials constituting the photoconductive layer 31 aremercury iodide, indium iodide, bismuth iodide, thallium iodide, andthallium bromide (TlBr), these results showed similar tendencies which,accordingly are considered to have a common phenomenon regarding thephotoconductive layer 31 of the heavy metal halide base. Further, in thecase the X-ray photoconductive material constituting the photoconductivelayer 31 are two or more kinds of composite system, and even in thecase, for example lead iodide and mercury iodide were film-formed bybinary vapor-deposition, similar effects have been recognized.

According to the present invention, since respective changes of thedark-current characteristic, the sensitivity characteristic, and theafterimage characteristic in the photoelectric converting layerirradiated with the radiation can be suppressed, the detectingsensitivity and the stability in this photoelectric converting layer canbe improved since at least one or more kinds of heavy metal halide(AB_(n):A=heavy metal, B=halogen, n=either one of 1, 2, and 3), and atleast one or more kinds of halogen (B₂) are respectively contained inthe photoelectric converting layer.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and representative embodiments shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventiveconcept as defined by the appended claims and their equivalents.

1. A radiation detector comprising: pixel electrodes which collectcharges; a photoelectric converting layer which is provided on the pixelelectrodes and which converts incident radiation into the charges, andwhich contains at least one or more kinds of heavy metal halide(AB_(n):A=heavy metal, B=halogen, n=either one of 1, 2, and 3) and atleast one or more kinds of halogen (B₂) respectively; and an electrodelayer which is provided on the photoelectric converting layer oppositeto the pixel electrodes.
 2. The radiation detector according to claim 1wherein the photoelectric converting layer contains at least one kind ofheavy metal (A).
 3. The radiation detector according to claim 1 whereinin the photoelectric converting layer, molar composition ratio (B/(nA))is 0.9or more and 1.1or less.
 4. The radiation detector according toclaim 1 wherein in the photoelectric converting layer, respectivecontents contained in the photoelectric converting layer of elementsbelonging to neighboring families to the heavy metal element and ofelements neighboring to the elements belonging to families in theneighboring periods before and after the heavy metal element is 10 wtppmor less.
 5. The radiation detector according to claim 1 wherein at leastone kind or more of heavy metal halide is at least one kind or more oflead iodide (PbI₂), mercury iodide (HgI₂), indium iodide (InI), thalliumiodide (TlI), and bismuth iodide (BiI₃), and the halogen is iodine (I).